Coal is the world's most abundant fossil fuel. However, coal has three major drawbacks: (1) Coal is a solid and is less easily handled and transported than fluidic or gaseous materials; (2) Coal contains compounds which, on burning, produce the pollutants associated with acid rain; and (3) Coal is not a uniform fuel product, varying in characteristics from region to region and from mine to mine.
In fossil fuels, the ratio of hydrogen atoms to carbon atoms is most important in determining the heating value per unit weight. The higher the hydrogen content, the more liquid (or gaseous) the fuel, and the greater its heat value. Natural gas, or methane, has a hydrogen-to-carbon ratio of 4 to 1 (this is the maximum); gasoline has a ratio of almost 2.2 to 1; petroleum crude about 2.0 to 1; shale oil about 1.5 to 1; and coal about 1 to 1.
The lignites, peats, and lower calorific value subbituminous coals have not had an economic use except in the vicinity of the mine site, for example, mine mouth power generation facilities. This is due primarily to the cost of shipping a lower Btu product as well as to the danger of spontaneous combustion because of the high content of volatile matter and high percentage of moisture which is characteristic of such coals. The risk of spontaneous combustion is increased by dehydration, even by the non-evaporation methods. Therefore, in order to secure stability of the dehydrated coal in storage and transportation, it has been necessary to cover the coal with an atmosphere of inert gas such as nitrogen or combustion product gas, or to coat it with crude oil so as not to reduce its efficiency as a fuel. However, these methods are not economical.
Waste coal has somewhat different inherent problems from those of the low-rank coals. Waste coal is sometimes referred to as a "non-compliance coal" because it is too high in sulfur per unit heat value to burn in compliance with the United States Environmental Protection Agency (EPA) standards. Other waste coal is too low in heat value to be transported economically. This coal represents not only an environmental problem (because it must be buried or otherwise disposed of), but also is economically unattractive.
The inefficient and expensive handling, transportation and storage of coal (primarily because it is a solid material) prevent coal from being an economically exportable product and cause the conversion of oil-fired systems to coal to be economically unattractive. Liquids are much more easily handled, transported, stored and fired into boilers.
Besides being difficult to transport, coal is a heterogeneous fuel, i.e, coal from different reserves has a wide range of characteristics and quality. Coal from one region (or even of a particular mine) cannot be efficiently combusted in boilers designed for coal from another source. Boilers and pollution control equipment must either be tailored to a specific coal or configured to burn a wide variety of material with a loss in efficiency.
The non-uniformity and transportation problems are compounded by the presence of combustion pollutants in coal, such as sulfur and nitrogen compounds which are thought to cause acid rain. The sulfur compounds are of two types: organic and inorganic (pyritic), both of which produce SO.sub.x. The fuel bound nitrogen, i.e., organic nitrogen in the coal, combusts to form NO.sub.x. Further, because of the non-uniformity of coal it combusts with "hot spots" which results in some of the nitrogen in the combustive air (air is 75% nitrogen by weight) being oxidized to produce NO.sub.x. This so-called "thermal NO.sub.x " has heretofore only been reduced by expensive boiler modification systems.
Raw coal cleaning has heretofore been available to remove inorganic ash and sulfur but has been unable to remove the organic nitrogen and organic sulfur compounds which, upon combustion, produce the SO.sub.x and NO.sub.x pollutants. Heretofore fluidized bed boilers, which require limestone as an SO.sub.x reactant, and scrubbers or NO.sub.x selective catalytic convertors (so-called combustion, and post-combustion clean air technologies) have been the main technologies proposed to alleviate these pollution problems. These devices clean the combustion and flue gas rather than the fuel and are tremendously expensive from both capital and operating standpoints, adding to the cost of power. This added power cost not only increases the cost of domestically produced goods, but also ultimately diminishing this nation's competitiveness with foreign goods. Moreover, operation of post-combustion pollution control equipment draws on the power generated in the plant, reducing saleable plant output. This inefficiency results in higher production of CO.sub.2 per unit of power available for sale. Carbon dioxide has been linked by some with the "greenhouse" effect, i.e., the warming of the earth's atmosphere.
It would, therefore, be advantageous to clean up the coal by removing the organic nitrogen (fuel nitrogen), as well as the organic sulfur while providing a uniform, highly reactive fuel which burns at a lower temperature, thereby reducing the production of thermal NO.sub.x.
In order to overcome some of the inherent problems with coal as a solid fuel, various methods have been proposed for converting coal to synthetic liquid or gaseous fuels. These liquefaction "synfuel" processes are capital intensive and require a great deal of externally supplied water and external hydrogen, i.e., hydrogen and water provided from other than the coal feedstock. The processes are also energy intensive in that most carbon atoms in the coal matrix are converted to hydrocarbons, i.e., no pure carbon. This differs markedly from merely "rearranging" existing hydrogen in the coal molecule as in hydrodisproportionation which hydrogenates certain carbon atoms at the expense of other carbon atoms.
Coal pyrolysis is a well-known process whereby coal is thermally volatilized by heating the coal out of contact with air. Different pyrolysis products may be produced by varying the conditions of temperature, pressure, atmosphere, and/or material feed. Traditional pyrolysis has produced very heavy hydrocarbon tars and carbon (char), with the liberation of hydrogen.
In prior art pyrolysis, as shown in FIG. 2, the pulverized coal is heated relatively slowly at low heating rates and for long residence times such that the coal molecule undergoes a slow decomposition at reaction rate "k.sub.1 " to yield "decomposition" products, primarily free radical hydrocarbon fragments. These "decomposition" products undergo a rapid recomposition or "condensation" reaction at reaction rate "k.sub.2 ", producing char and dehydrogenated hydrocarbons, and liberating hydrogen. The decomposition reaction is not desirable in a refining type process because it liberates valuable hydrogen instead of utilizing it to upgrade the hydrocarbon products. As shown in FIG. 2, when heating is slower such that k.sub.1 (relatively slow reaction rate) and k.sub.2 (relatively more rapid reaction rate) overlap, the dehydrogenation of the decomposition product, i.e., condensation reaction, is predominant. It is believed that when the decomposition reaction take place slowly, this reaction and the condensation reaction will take place within the coal particle where there is little hydrogen present to effect the hydrogenation reaction. This results in the production of heavy tar-like liquids of limited utility.
Prior art hydropyrolysis of bituminous and subbituminous coals of various ranks attempted to hydrogenate decomposition products through the use of external hydrogen. This process, sometimes called "partial liquefaction", has been carried out in both the liquid and gaseous phases. As used herein, "partial liquefaction" is meant to include all thermally based coal conversion processes, whether catalyzed or not, wherein a partial pressure of hydrogen is present. In order to promote hydrogenation, more stringent reaction conditions were required, reducing the economic viability. Examples of such processes are disclosed in U.S. Pat. Nos. 4,704,134; 4,702,747; and 4,475,924. In such processes, coal is heated in the presence of hydrogen or a hydrogen donating material to produce a carbonaceous component called char and various hydrocarbon-containing oil and gas components. The most economical of these processes take place under milder conditions; however, these processes have had only limited success. As in pyrolysis, if the heating rates are not rapid, the decomposition material remains inside the coal particle and can not be hydrogenated by external hydrogen without use of extreme temperatures and pressures. This substantially increases the cost and effectively makes these processes "liquefaction" processes.
In "liquefaction" processes, coal is treated with hydrogen to produce petroleum substitutes. These processes, which have been known for many years, have typically mixed crushed coal with various solvents, with or without catalysts; heated the mixture to reaction temperature; and reacted the coal and hydrogen at high pressure and long residence times. "Liquefaction" processes require high pressure, usually above 2,000 psig; require long reaction residence times, 20 minutes to about 60 minutes; consume large quantities of expensive externally generated hydrogen; and produce large amounts of light hydrocarbon gases. Solvent addition and removal, catalyst addition and removal, high pressure feed system, high pressure long residence time reactors, high hydrogen consumption, and high pressure product separation and processing have made these processes uneconomical in today's energy market.
A particular type of coal hydropyrolysis, flash hydropyrolysis, is characterized by a very short reactor residence time of the coal. Short residence time (SRT) processes are advantageous in that the capital costs are reduced because the feedstock throughput is so high. In SRT processes, high quality heat sources are required to effect the transformation of coal to char, liquids and gases.
In many processes, hydrogen is oxidized within the reactor to gain the high quality heat. However, the oxidation of hydrogen in the reactor not only creates water but also reduces the hydrogen available to hydrogenate hydrocarbons to produce higher quality fuels. Thus, in prior art processes, either external hydrogen is required or the product is degraded because valuable hydrogen is converted to water.
The prior art methods of deriving hydrogen for hydropyrolysis or liquefaction are by: (1) purchasing or generating external hydrogen, which is very expensive; (2) steam-methane reforming followed by shift conversion and CO.sub.2 removal as disclosed in a paper by J. J. Potter of Union Carbide; or (3) char gasification with oxygen and steam followed by shift conversion and CO.sub.2 removal as disclosed in a paper by William J. Peterson of Cities Service Research and Development Company.
All three of these hydrogen production methods are expensive, and a high temperature heat source such as direct O.sub.2 injection into the hydropyrolysis reactor is still required to heat and devolatilize the coal. In the prior art processes, either carbon (char) is gasified by partial oxidation such as in a Texaco gasifier (U.S. Pat. No. 4,491.456 to Schlinger and U.S. Pat. No. 4,490,156 to Marion et al.), or oxygen was injected directly into the reactor. One such system is disclosed in U.S. Pat. No. 4,415,431 (1983) of Matyas et al. When oxygen is injected directly into the reactor, it preferentially combines with hydrogen to form heat and water. Although this reactor gives high-quality heat, it uses up hydrogen which is then unavailable to upgrade the hydrocarbons. This also produces water that has to be removed from the reactor product stream and/or floods the reactor. Additionally, the slate of hydrocarbon co-products is limited.
Flash hydropyrolysis has additional drawbacks in that the higher heating rates needed for short residence times tend to thermally hydrocrack and gasify the material at lower pressures. This gasification reduces liquid yield and available hydrogen. Thus, attempts to increase temperature to effect flash reactions tended to increase the hydrocracking of the valuable liquids to gases.
Thus, it would be advantageous to have a means for producing: (1) a high-quality heat for volatilization, (2) hydrogen, (3) other reducing gases prior to the reaction zone without producing large quantities of water and without using up valuable hydrogen, and (4) high quality liquid hydrocarbons.
In U.S. Pat. Nos. 4,671,800; 4,658,936; 4,832,831; and 4,878,915, it is disclosed that coal can be subjected to pyrolysis or hydropyrolysis under certain conditions to produce a particulate char, gas and a liquid organic fraction. The liquid organic fraction is rich in hydrocarbons, is combustible, can be beneficiated and can serve as a liquid phase for a carbonaceous fluidic fuel system. The co-product distribution, for example, salable hydrocarbon fractions such as BTX and naphtha, and the viscosity, pumpability and stability of the fluidic fuel when the char is admixed with the liquid organic fraction are a function of process and reaction parameters. The rheology of the fuel system is a function of solids loading, sizing, surfactants, additives, and oil viscosity.
Common volatilization reactors include the fluidized bed reactor which uses a vertical upward flow of reactant gases at a sufficient velocity to overcome the gravitational forces on the carbonaceous particles, thereby causing movement of the particles in a gaseous suspension. The fluidized bed reactor is characterized by large volumes of particles accompanied by long, high-temperature exposure times to obtain conversion into liquid and gaseous hydrocarbons. Thus, this type of reactor is not very conducive to short residence time (SRT) processing and may produce a large quantity of polymerized (tar-like) hydrocarbon co-products.
Another common reactor is the entrained flow reactor which utilizes a high-velocity stream of reactant gases to impinge upon and carry the carbonaceous particles through the reactor vessel. Entrained flow reactors are characterized by smaller volumes of particles and shorter exposure times to the high-temperature gases. Thus, these reactors are useful for SRT-type systems.
In one prior art two-stage entrained flow reactor, a first stage is used to react carbonaceous char with a gaseous stream of oxygen and steam to produce hydrogen, oxides of carbon, and water. These products continue into the second stage where volatile-containing carbonaceous material is fed into the stream. The carbonaceous feed reacts with the first-stage gas stream to produce liquid and gaseous hydrocarbons, including large amounts of methane gas and char.
Prior art two-stage processes for the gasification of coal to produce primarily gaseous hydrocarbons include U.S. Pat. Nos. 4,278,445 to Stickler; U.S. Pat. No. 4,278,446 to Von Rosenberg, Jr.; and U.S. Pat. No. 3,844,733 to Donath. U.S. Pat. No. 4,415,431 issued to Matyas et al. shows use of char as a carbonaceous material to be mixed with oxygen and steam in a first-stage gasification zone to produce a synthesis gas. Synthesis gas, along with additional carbonaceous material, is then reacted in a second-stage hydropyrolysis zone wherein the additional carbonaceous material is coal to be hydropyrolyzed.
U.S Pat. No. 3,960,700 to Rosen describes a process for exposing coal to high heat for short periods of time to maximize the production of desirable hydrocarbons.
One method of terminating the volatilization reaction is by quenching the products either directly with a liquid or gas, or by use of a mechanical heat exchanger. In some cases, product gases or product oil are used. Many reactors, including those for gasification have employed a quench to terminate the volatilization reaction and prevent polymerizing of unsaturated hydrocarbons and/or gasification of hydrocarbon products. Some have employed intricate heat-exchange quenches, for example, mechanical devices to attempt to capture the heat of reaction. One such quench scheme is shown in U.S. Pat. No. 4,597,776 issued to Ullman et al. The problem with these mechanical quench schemes is that they introduce mechanical heat-exchanger apparatus into the reaction zone. This can cause tar and char accumulation on the heat-exchanger devices, thereby fouling the heat exchanger.
Thus, if the coal has a hydrogen-to-carbon ratio of 1, and if the hydrogens on half the carbons could be transferred or "rearranged" to the other half of the carbons, then the result would be half the carbons with 0 hydrogens and half with 2 hydrogens. The first portion of carbons (with 0 hydrogens) is char; the second portion of carbons (with 2 hydrogens) is a liquid product similar to a petroleum fuel oil. If this could be accomplished using only hydrogen inherent in the coal, i.e., no external hydrogen source, then the coal could be refined in the same economical manner as petroleum, yielding a slate of refined hydrocarbon products and char.
In a further attempt to alleviate the problems of transporting the energy from non-uniform, solid coal to the end use power generation facility, three methods have been suggested to "co-generate" electricity and another coal-derived product. In all three types, the co-generation facility is usually placed at mine mouth, or in close proximity thereto. In the first type, the coal is processed to create synthetic gas or liquid fuel which is fed to a gas turbine that generates electricity. The turbine is exhausted to a heat exchange which produces high temperature process steam for use as chemical process heat or the like. In a second type, coal is burned directly in a steam boiler to produce steam which drives a turbine. The turbine generates electricity and the exhaust is used as process heat for chemical processes or the like. The third type, the so-called "combined cycle cogeneration system", involves the production from coal of synthetic gas ("syngas") which is combusted in a gas turbine to produce electricity. The exhaust gas is heat exchanged to produce steam which drives a second electric generating turbine. The exhaust from this turbine is then used to produce process heat for a chemical plant or the like. Cogeneration facilities using the syngas approach have not been altogether successful since this process requires the conversion of all or substantially all of the coal to liquid or gas, which is energy intensive and expensive. Further, as with "synfuels", the product can be a transportation fuel which is easily pipeline transportable and too expensive to be utilized in stationary units. Another disadvantage has been that the electrical facility is limited by the marketability of the process heat generated. Thus, the electric generating facility must operate in conjunction with a chemical plant or some similar process heat user. Additionally, most power generating stations are based upon economies of scale in the 400 to 500 MW range. This has proven expensive in that the capital costs for excess capacity for combined cycle facilities are not justified unless the entire plant is utilized fully. The size of the plant also limits the site available for cogeneration facilities.
In short, the U.S. energy scene has focused on a number of individual solutions to a many-faceted problem. A fuel "systems" approach is necessary to fully utilize the nation's substantial coal reserves. By forming a modular co-generating system wherein waste heat is used to produce petroleum substitutes which can be readily transported by pipeline, tanker train or the like, all of the fuel is utilized efficiently and effectively, yielding flexibility in use and distribution.
It would be highly advantageous to first refine the coal to extract high quality, value-added hydrocarbon liquid products, which are useful as petroleum substitutes and/or chemical feedstocks, and use at least part of the remaining (char) carbons to produce methanol in a once-through process by gasification with oxygen to produce CO and H.sub.2 from water, with the remaining gas being used for electric turbine fuel.
Methanol, when combined with ethanol and/or gasoline, creates a clean burning, motor vehicle fuel. Methanol is also a feedstock for producing methyl tertiary-butyl ether (MTBE), another oxygenated fuel additive which is currently used in cities such as Denver and Phoenix to reduce transportation caused carbon monoxide air pollution. This is important in this nation's campaign against pollution. Therefore, an inexpensive method of production of methanol from coal would be advantageous.
Making methanol from coal is well-known. In accordance with this process, methanol is made directly from coal and steam to initially form carbon monoxide and hydrogen in accordance with equation I: EQU HOH (steam)+C (coal).fwdarw.CO+H.sub.2 I.
A portion of the gas is subjected to the shift reaction with steam to produce additional hydrogen in accordance with equation II: EQU CO+HOH (steam).rarw..fwdarw.CO.sub.2 +H.sub.2 II.
The CO.sub.2 is scrubbed from the gaseous product leaving primarily hydrogen. The hydrogen is admixed with gaseous products of equation I to produce a gas having a desired ratio of hydrogen to carbon monoxide from which methanol and similar products are synthesized catalytically.
In the methanol synthesis plant, carbon monoxide and hydrogen are combined to produce methanol. These constituents have heretofore only been economically available from natural gas. The synthesis of methanol is described in pages 370-398 of Vol. 13 of the KIRK-OTHMER ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, second edition, Anthony Standin, editor, Interscience Publishers, New York, 1969, Vol. 5. The carbon monoxide and hydrogen are controlled in a ratio and temperature-pressure combination to obtain maximum yields of the methanol fuel product. Other methods for methanol synthesis at lower temperatures and pressures are also known as, for example, the ICI low pressure process described in "Here's How ICI Synthesizes Methanol at Low Pressure", Oil and Gas Journal, Vol. 66, pp. 106-9, Feb. 12, 1968. The problem with these prior art methods is that the production of the starting materials, i.e., CO and H.sub.2, from coal was very expensive.
Thus, it would be highly advantageous to have a process which uses only coal and a small quantity of external water to easily and efficiently produce large quantities of methanol; syngas for turbine electrical generation; a clean burning fluidic boiler fuel; and a slate of co-products useful as petroleum substitutes or chemical feedstocks including benzene, toluene, xylene (BTX); ammonia; sulfur; naphtha; fuel oil; and the like.
Further, it would be highly advantageous to have a process for refining coal wherein short residence times and internally generated hydrogen are used in mild conditions to efficiently produce hydrocarbon liquids and to efficiently and economically produce alternative transportation fuels, including oxygenated fuels, as well as co-generating electricity.
Finally, it would be advantageous to co-produce value-added petroleum substitutes and chemical feedstocks by refining coal and utilizing the hot process carbon (char) with process water to produce methanol and fuel to spin turbines.
Thus, it would be highly advantageous to have a co-generating system which would produce electricity while utilizing the process heat in the production of petroleum substitutes and oxygenated fuels from coal using no external water, preferably in a combined cycle configuration.